1. Field of the Invention
The present invention relates to a wire-cut electric discharge machining method by which the machining of a workpiece adapted to have a machined shape including a tapered surface with a continuously changing cone angle, the machining of a workpiece adapted to have different machined shapes at the upper and lower surfaces thereof, or the like can be carried out.
2. Prior Art
There have been well-known wire-cut electric discharge machining methods which can carry out the machining of a workpiece adapted to have a tapered surface with a continuously changing cone angle, such as those shown in FIG. 7 illustrating a workpiece having machined circular shapes at the upper and lower surfaces of a workpiece which are eccentric from each other, and as shown in FIG. 8 illustrating a workpiece having machined rectangular shapes at the upper and lower surfaces of a workpiece which are twisted with respect to each other, and the machining of a workpiece adapted to have different machined shapes at the upper and lower surfaces. For example, Japanese Laid-Open Patent No. 60-56842 discloses such a kind of wire-cut electric discharge machining methods.
However, the above-mentioned prior art suffers from a problem such that the arrangement of an apparatus and the content of a process (such as a machining program or the like) are complicated since interpolation is made at the position of a wire guide which usually circumscribes a complicated locus during machining and since shapes at the upper and lower surfaces of a workpiece are each decomposed into micro line segments which are then approximated to arcs or straight lines in order to carry a machining process, and further this process is carried out inevitably with a low degree of accuracy.
More specifically, if the workpiece is cut with the use of the above-mentioned prior art machining method along a line segment extending from points S1 to Sn on the upper surface of the workpiece and a line segment extending from points P1 to Pn on the lower surface of the workpiece, several points S1, S2, . . . Sn, P1, P2, . . . Pn are selected on both line segments, and then the positions U1, U2, . . . Un, L1, L2 . . . Ln of the upper and lower guides are obtained, corresponding to these points S1, S2 . . . Sn, P1, P2 . . . Pn. Then, interpolation is made between each adjacent points P1 and P2, U1 and U2, P2 and P3, U2 and U3, . . . , and thereafter, the upper and lower guides are moved respectively along interpolating straight lines or curves therebetween. However, in the case of a machining process in which machined shapes at the upper and lower surfaces of a workpiece are different from each other, the above-mentioned points S1, S2, . . . Sn, P1, P2, . . . Pn should be selected in a large number in order to make distances between the adjacent points extremely short so as to ensure a high degree of accuracy for surfaces between the adjacent points since the loci of the upper and lower wire guides circumscribe curves which are more complicated than the loci on the upper and lower surfaces of a workpiece to be machined. Accordingly, a machining program for the above-mentioned prior art machining method should have had a large number of steps which occupy a substantial area of a memory device in an NC apparatus and which complicates the preparation of the machining program.